1. Field of the Invention
This invention relates to the manufacture of pulp from lignocellulosic material, and in particular wood pulp from softwood. More particularly, this invention relates to a three-stage process for the treatment of lignocellulosic material, in particular softwood chips to produce pulp comprising, first treating the lignocellulosic material with a cyanide ion-containing solution, second pulping in an alkaline pulping stage, and third treating with oxygen.
2. Description of the Prior Art
As is well known, any improvement in yield obtained from the pulping of wood chips without serious loss of physical properties of the resulting pulp is important to the paper industry. In fact, the increase in yield of the pulping process by as much as 1% or 2% is significant and is highly desirable if the economic costs are justified. Consequently, numerous attempts have been made to improve pulp yields. These attempts include various chemical processes for treating lignocellulosic material, as well as, varying digesting parameters of these processes.
Lignocellulosic materials (such as wood from coniferous and deciduous species of trees) contain lignin, cellulose, and hemicelluloses. The lignin and hemicelluloses, such as gluco-mannan chains in softwood or glucurono-xylan chains in hardwood, are located both in the cell wall of the fiber and in the inter-fiber spaces, generally termed the middle lamellae. It is highly desirable to retain the hemicelluloses on the surface of the fiber, in which case they can influence the bonding properties of the fibers; and they are also retained in the cell wall of the fiber, improving flexibility and thereby the papermaking properties of the pulp. In order to retain the hemicelluloses, various chemical processes aiding the retention of the hemicelluloses have been proposed. For example, one particularly effective process which encompasses the treatment of the lignocellulosic material prior to digesting with an alkaline pulping solution has been set forth in U.S. Pat. No. 3,532,596 to Bills et al. In the Bills et al. patent, incorporated herein by reference, there is disclosed the reaction of a cyanide ion-containing solution with lignocellulosic materials wherein the cyanide ion in the presence of hydroxyl ions reacts to prevent the peeling of the carbohydrate chain. Although the process set forth in the Bills et al. patent is certainly significant, it is desirable to obtain even higher pulp yields than can be affored by the process of the Bills et al. patent.
It is also to be noted that treatment of pulp through contact with sodium hydroxide in the presence of an excess of oxygen or, in other words, a post pulping treatment with oxygen and the like, has been set forth in U.S. Pat. No. 3,384,583 to Robert et al. and U.S. Pat. No. 3,691,008 to Worster et al. The Robert et al. and Worster et al. patents disclose alkaline pulping processes, which after a conventional alkaline pulping stage the pulps are subjected to oxygenation in a second or post-treatment step to obtain slightly higher unbleached pulp yields, higher unbleached pulp brightness and easier beating and bleachability.
It is, therefore, the general object of this invention to provide a three stage process for pulping lignocellulosic material, in particular softwood chips. Another object of this invention is to provide a three stage process including, a cyanide treatment stage, followed by an alkali metal digestion stage, that followed by an oxygenation stage to provide quality pulps in higher yields.
Other objects, features and advantages of this invention will be evident from reading the foregoing detailed description of the preferred embodiments and the accompanying drawing.